Detroit Diesel Fault Code
Free · no signupSPN 520371 FMI 16
MIL On / Elevated DEF Consumption
System
Emissions
Symptom
MIL On / Elevated DEF Consumption
Diagnosis & fix steps
LOGIC: SCR Closed Loop Control at Maximum Limit (Multiple Sources) / High DEF Delivery.
ACTION: Test DEF concentration with refractometer (32.5% spec). Run a DEF quantity test via DiagnosticLink (spec: 108–132 mL). Perform a parked SCR efficiency test to reset tracking.
Red Seal field insights
Control Logic: The ACM continually compares NOx levels into and out of the SCR. If conversion efficiency drops, it aggressively ramps up DEF dosing. If it maxes out the dosing map volume and NOx is still high, this code sets.
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